1. Field of the Invention
The present invention relates to bonded composite open mesh structural textiles primarily designed for use as structural load bearing elements in earthwork construction applications such as earth retention systems (in which the load bearing element is used to internally reinforce steeply inclined earth or construction fill materials to improve their structural stability), foundation improvement systems (in which the load bearing element is used to support and/or internally reinforce earth or foundation fill materials to improve their load bearing capacity), pavement improvement systems (in which the load bearing element is used to internally reinforce flexible pavements or to support rigid modular paving units to improve their structural performance and extend their useful service lives) or erosion protection systems (in which the load bearing element is used to confine or internally reinforce earth or construction fill materials in structures which are subject to erosion or which prevent erosion elsewhere by dissipating wave energy in open water). While the materials of this invention have many other diverse applications, they have been primarily designed to embody unique characteristics which are important in engineered earthwork construction and particular emphasis is placed on such uses throughout this application.
2. Description of the Prior Art
Geogrids and geotextiles are polymeric materials used as load bearing, separation or filtration elements in many earthwork construction applications. There are four general types of materials used in such applications: 1) integrally formed structural geogrids; 2) woven or knitted textiles; 3) open mesh woven or knitted textiles (which are generally configured to resemble and compete with integrally formed structural geogrids); and 4) non-woven textiles.
Integrally formed structural geogrids are formed by extruding a flat sheet of polymeric material, punching apertures in the sheet in a generally square or rectangular pattern and then uniaxially or biaxially stretching the apertured sheet, or by extruding an integrally formed mesh structure which constitutes a sheet with apertures in a generally square or rectangular pattern and then uniaxially or biaxially stretching the apertured sheet. Woven or knitted textiles are formed by mechanically interweaving or interlinking polymeric fibers or fiber bundles with conventional textile weaving or knitting technologies. Open mesh woven textiles are formed in this same manner and are normally coated in a subsequent process. Non-woven textiles are formed by various techniques including overlaying and mechanically entangling polymeric fibers, generally by needling, and in some processes the entangled polymeric fibers are then re-oriented in a biaxial stretching process, calendered and/or heat fused.
Integrally formed structural geogrids are well known in the market and are an accepted embodiment in many earthwork construction applications. Open mesh woven or knitted textiles, generally characterized and marketed as textile geogrids, compete directly with integrally formed structural geogrids in many applications and have also established an accepted position in earthwork construction markets. Competition between either of these "geogrid" materials and conventional woven or knitted textiles is less frequent. Woven or knitted textiles with low basis weight tend to be used in separation and filtration applications. Woven or knitted textiles with high basis weight tend to be used in load bearing applications which are tolerant to the load-elongation properties of such materials and which can beneficially use the high ultimate tensile strength of such materials. Non-woven textiles are generally subject to very high elongation under load and are not normally used in load bearing earthwork construction applications. Competition between either of the "geogrid" materials and non-woven textiles is negligible.
The characteristics of integrally formed structural geogrids and open mesh woven or knitted textiles are significantly different in several respects. The integrally formed materials exhibit high structural integrity with high initial modulus, high junction strength and high flexural and torsional stiffness. Their rigid structure and substantial cross sectional profile also facilitate direct mechanical keying with construction fill materials, with contiguous sections of themselves when overlapped and embedded in construction fill materials and with rigid mechanical connectors such as bodkins, pins or hooks. These features of integrally formed structural geogrids provide excellent resistance to movement of particulate construction fill materials and the integrally formed load bearing elements relative to each other, thereby preserving the structural integrity of foundation fill materials or preventing pull out of the embedded load bearing elements in earth retention applications.
Integrally formed structural geogrids interact with soil or particulate construction fill materials by the process of the soil or construction fill materials penetrating the apertures of the rigid, integrally formed geogrid. The result is that the geogrid and the soil or construction fill materials act together to form a solid, continuously reinforced matrix. Both the longitudinal load bearing members and the transverse load bearing members and the continuity of strength between the longitudinal and the transverse load bearing members of the geogrid are essential in this continuous, matrix-like interlocking and reinforcing process. If the junction between the longitudinal and the transverse load bearing members fails, the geogrid ceases to function in this manner and the confinement and reinforcement effects are greatly reduced. Their rigid structure also facilitates their use over very weak or wet subgrades where placement of such load bearing materials and subsequent placement of construction fill materials is difficult.
The open mesh woven or knitted materials exhibit higher overall elongation under load, lower initial modulus, softer hand and greater flexibility. With sufficient increase in the number of fibers or fiber bundles comprising their structure they are capable of achieving higher ultimate tensile strength than integrally formed structural geogrids. However, they also exhibit low junction strength which limits their effectiveness in direct mechanical keying with construction fill materials, with contiguous sections of themselves when embedded in construction fill materials or with rigid mechanical connectors. As a result, such materials are primarily used in applications which rely on a frictional interface with construction fill materials to transfer structural loads to the load bearing element and users of such materials also avoid applications which involve load bearing connections with rigid mechanical connectors. Also, their low flexural and torsional stiffness limit their practical usefulness and performance in certain earthwork applications such as construction over very weak subgrades or construction fill reinforcement in foundation improvement applications.
The attributes which are most pertinent to the use of polymeric materials in structural load bearing earthwork construction applications are:
(a) the load transfer mechanism by which structural forces are transferred to the load bearing element, PA1 (b) the load capacity of the load bearing element; PA1 (c) the structural integrity of the load bearing element when subjected to deforming forces in installation and use; and PA1 (d) the resistance of the load bearing element to degradation (i.e., loss of key properties) when subject to installation or long term environmental stress. PA1 (a) its load transfer mechanism (specifically its suitability for direct mechanical keying with construction fill materials, with contiguous sections of itself when overlapped and embedded in construction fill materials and with rigid mechanical connectors such as bodkins, pins or hooks); PA1 (b) its load capacity (specifically its initial modulus, i.e., its resistance to elongation when initially subject to load); PA1 (c) its structural integrity (specifically its junction strength and its flexural and torsional stiffness); and PA1 (d) its durability (specifically its resistance to degradation when subject to installation and long term environmental stress).
The limitations which open mesh woven or knitted textiles exhibit with respect to the first three attributes listed above primarily result from a lack of rigidity and tautness in the fibers or fiber bundles in the junction zones of these materials in which many separate fibers or fiber bundles are interlinked, interwoven or entangled in a manner which is characteristic of a woven or knitted structure and which does not cause the load bearing fibers or fiber bundles to be either taut or dimensionally stable relative to each other. The limitations which such materials exhibit with respect to the fourth attribute listed above primarily result from degradation of their coating materials and separation of such coating materials from the load bearing fibers.
Attempts have been made to dimensionally stabilize and protect the fibers or fiber bundles in the junction zones of open mesh woven or knitted textiles. For instance, such textiles are normally coated with another material such as polyvinylchloride after the principal textile structure is formed on a weaving or knitting loom. This technique improves the dimensional stability of the fibers or fiber bundles in the junction zone to some extent and also provides some protection from abrasion to the fibers throughout the textile. However, this technique has not delivered sufficient junction strength or sufficient initial modulus to enable such materials to be functionally comparable to integrally formed structural geogrids or to be directly competitive with integrally formed structural geogrids in certain demanding earthwork construction applications which require or benefit from load transfer by direct mechanical keying or high initial modulus or high structural integrity or stiffness in the load bearing element. The protective coatings also tend to degrade and separate from the load bearing fibers, thereby reducing their effectiveness in providing long term resistance to environmental degradation of the load bearing fibers and also creating a potential shear failure surface at the interface between the load bearing fibers and the coating material.